MOLECULAR CHARACTERIZATION OF COAT PROTEIN GENES OF SEROLOGICALLY DISTINCT ISOLATES OF POTATO-VIRUS-Y NECROTIC STRAIN

The coat protein (CP) genes of two potato virus Y necrotic isolates (N 27 and a mutant strain N27-92), which differed in their reactivity to a monoclonal antibody (mab), were characterized. Both isolates could b e detected by mab 4E7, but mab VN295.5 selectively reacted to N27 and not to N27-92. The CP genes of both isolates coded for 267 amino acids with similar to 99.0% identity at both the nucleotide and the amino a cid levels. Nucleotide sequence comparison indicated five substitution s in N27-92 compared with N27. Three of these changes resulted in subs titution of amino acids. Two transitions (A-G) in N27-92 changed threo nine to alanine and lysine to arginine at positions 7 and 55, respecti vely, whereas a A-->T transversion changed asparagine to isoleucine at position 27. The surface probability curves of both the isolates coul d almost be superimposed, except at amino acid positions 7 and 27. Sin ce amino acid substitution at position 55 is conservative, changes fro m polar to hydrophobic amino acids (threonine-->alanine and asparagine -->isoleucine) at positions 7 and 27 might have changed the epitope(s) of N27-92, abolishing its detection by mab VN295.5.